The present invention relates to a gradation correction circuit for correcting a gradation of a video signal, and a xcex3 correction apparatus using the gradation correction circuit.
In recent years, digital video display apparatuses such as an LCD and a PDP have been widely spread in addition to a conventional CRT. In these video display apparatuses, a gradation of a level of an output signal with relative to a level of an input level is not linear, and the video display apparatus have inherent input output characteristics, respectively. Therefore, in order to obtain optimum images in accordance with the inherent display characteristics of these display apparatus, a reverse correction corresponding to display characteristics of each display apparatus is performed by a gradation correction circuit, and an output signal of the gradation correction circuit is inputted to the display apparatus. By using such gradation correction circuit, the input output characteristics between the input to the gradation correction circuit and the output from the display apparatus become linear.
A conventional gradation correction circuit has such a construction that correction characteristics is stored in storage devices such as a RAM or a ROM, and correction data in accordance with the level of the input signal is read out from a memory to be outputted as corrected signal.
Another conventional gradation correction circuit has such a construction that the correction characteristics is subjected to a linear approximation employing a broken line, the broken line is operated by an arithmetic circuit constituted by multipliers and adders, and outputs of the arithmetic circuit corresponding to the input level are selected to provide a corrected signal.
FIG. 6(a) is a block diagram illustrating a structure of a conventional gradation correction circuit.
In FIG. 6(a), the conventional gradation correction circuit includes arithmetic circuits 700, 701 and 702 each comprising a multiplier and an adder, which arithmetic circuit outputs values of the respective sections of the broken line in accordance with the input signal X. A selection circuit 703 which selects one among the outputs of the arithmetic circuits 700, 701 and 702 is provided to output the corrected correction output signal Y.
The conventional gradation correction circuit thus constructed presents the input output characteristics as shown in FIG. 6(b).
However, in the gradation correction circuit which stores the correction characteristics in a memory such as a RAM or a ROM, it is necessary to memorize the values of the outputs corresponding to the inputs stored in a memory in a form of a lookup table, and all the points are required to be stored in a memory. Therefore, when it is attempted to increase the bit number in order to increase the gradation, a larger memory capacity is required.
In the conventional gradation correction circuit shown in FIG. 6, a multiplier and an adder are required for each straight line of the broken line. Therefore, when the number of the straight lines of the broken line is increased so as to perform detailed correction employing the broken line, i.e., correction with high precision, the number of multipliers increases and the circuit size is increased.
The present invention is made to solve the above problems, and has for its object to provide a gradation correction circuit and a xcex3 correction apparatus which can perform gradation correction without using a memory storing a lookup table, and which can perform correction with high precision on the basis of an input output characteristic represented by a broken line without increasing a circuit scale.
According to a first aspect of the present invention, a gradation correction circuit which performs gradation correction of an input video signal on the basis of input output characteristics represented by a broken line comprises: arithmetic means for operating output values corresponding to respective broken points of the broken line representing input output characteristics of a display apparatus in a vertical blanking period of an input video signal; a level detection circuit for identifying a size relation between each broken line point which is an input value corresponding to each broken point, and an input video signal, and operating a difference between the input video signal and each broken line point in a video signal period; correction means for selecting a gradient of the broken line on the basis of an identification result from the level detection circuit, and for performing gradation correction of the input video signal on the basis of the gradient of a selected broken line, the output value being calculated by the arithmetic means, and the difference calculated by the level detection circuit.
Therefore, gradation correction with high precision can be realized without increasing a circuit scale, or without using a memory storing a lookup table.
According to a second aspect of the present invention, in the gradation correction circuit of the first aspect, the arithmetic means and the correction means share a number of circuits.
Therefore, gradation correction with high precision can be realized without increasing a circuit scale, or without using a memory storing a lookup table.
According to a third aspect of the present invention, a gradation correction circuit for a display apparatus which performs gradation correction of an input video signal on the basis of input output characteristics represented by a broken line comprises: a first selector for selecting one among plural gradients of a broken line representing input output characteristics of a display apparatus; a difference arithmetic unit for calculating the difference between the plural broken line points which are the input values corresponding to the broken points of the broken line; a second selector for selecting one among the difference values calculated by the difference arithmetic unit; a level detection circuit for receiving the broken line points and the input video signal as inputs, identifying between which broken line points the input video signal lies, and calculating a difference between the input video signal and the broken line point; a third selector for selecting one between the difference value calculated by the level detection circuit and the difference value selected by the second selector; a multiplier for multiplying the gradient of the broken line selected by the first selector and the difference value selected by the third selector; an integrator for successively adding the values, which are successively outputted from the multiplier; a register for holding the output of the integrator as the output value corresponding to each broken point; an adder for adding the output value from the register and the output from the multiplier; and a control circuit for controlling the first through third selectors and the register on the basis of the identification result from the level detection circuit and the vertical blanking pulse, wherein, when the start of the vertical blanking period is detected by the vertical blanking pulse, the control circuit outputs a selection signal at a predetermined timing, thereby successively instructing the first and second selectors to select the gradients of the broken line and the difference values, respectively, and instructing the third selector to select the difference value selected by the second selector, and, when the start of the video signal period is detected by the vertical blanking pulse, the control circuit instructs the third selector to select the difference value calculated by the level detection circuit, and instructs the first selector to select the gradient of the broken line corresponding to the identification result from the level detection circuit as well as instructs the register to output the value corresponding to the identification result, on the basis of the identification result.
Therefore, gradation correction with high precision can be realized without increasing a circuit scale, or without using a memory storing a lookup table.
According to a fourth aspect of the present invention, the gradation correction circuit further comprises: a video detection control circuit for setting the broken line points and the gradients of the broken line on the basis of the input video signal.
Therefore, correction characteristics can be changed in real time, and an improvement in an image quality or the like can be effectively performed.
According to a fifth aspect of the present invention, a xcex3 correction apparatus comprises three gradation correction circuits, and a first gradation correction circuit performs gradation correction of an R signal, a second gradation correction circuit performs gradation correction of a G signal, a third gradation correction circuit performs gradation correction of a B signal, and correction characteristics of the first through third gradation correction circuits are set for each RGB signal.
Therefore, effective video correction of the input video signal can be performed.